Interrogators used "waterboarding" on three men shortly after the September 11 attacks, the CIA admitted today, naming for the first time the victims of a technique widely perceived as torture.

The men subjected to waterboarding, which simulates drowning, were al-Qaida suspects Khalid Sheikh Mohammed, Abu Zubaydah and Abd al-Rahim al-Nashiri, the CIA director, Michael Hayden, told the US Congress.

"We used it against these three detainees because of the circumstances at the time," Hayden said. "There was the belief that additional catastrophic attacks against the homeland were inevitable. And we had limited knowledge about al-Qaida and its workings. Those two realities have changed."

Hayden told the senate intelligence committee that Mohammed - the alleged mastermind of the 9/11 attacks - and the other two men were subject to waterboarding in 2002 and 2003.

"The circumstances are different than they were in late 2001, early 2002," Hayden said, adding that he opposed limiting the CIA to interrogation techniques permitted in the US Army field manual, which bans waterboarding.

Hayden told the committee that fewer than 100 people had been held in the CIA's terrorism detention and interrogation programme, with less than one-third subjected to "coercive" techniques.

The CIA said in December that it had destroyed videotapes depicting the interrogations of Zubaydah and Nashiri, prompting a justice department investigation. The tapes were destroyed as Congress moved to pass a ban on inhumane interrogations and a prosecutor is investigating whether US intelligence officials broke the law or violated court orders in destroying the tapes.

In waterboarding, the victim's mouth is covered and water poured over his face, making the victim feel as if they are drowning.

"Waterboarding taken to its extreme, could be death - you could drown someone," McConnell acknowledged. He said waterboarding remains a technique in the CIA's arsenal, but it would require the consent of the president and legal approval of the attorney general.

At the same hearing, the US director of national intelligence said the Taliban, which was overthrown in Afghanistan in late 2001, has expanded its operations into once-peaceful areas of western Afghanistan and around the capital, Kabul, despite the death or capture of three top commanders in the last year.

McConnell also said al-Qaida maintains a "safe haven" in Pakistan's tribal areas, where the group is able to stage attacks supporting the Taliban in neighbouring Afghanistan.

The Pakistani tribal areas provide al-Qaida "many of the advantages it once derived from its base across the border in Afghanistan, albeit on a smaller and less secure scale", allowing militants to train for strikes in Pakistan, the Middle East, Africa and the US, McConnell said.

"Al-Qaida remains the pre-eminent threat against the United States, both here at home and abroad," McConnell said, even though the terror network had suffered setbacks in Iraq.

He expressed concern that al-Qaida in Iraq is shifting its focus elsewhere in the region.

"They may deploy resources to mount attacks outside the country," McConnell said, although fewer than 100 terrorists have moved to establish cells in other countries.

McConnell also told the senate panel that US officials believe that Osama bin Laden is hiding in Pakistan's tribal areas. A report released in London said nearly 400 militant groups now operate around the world and the greatest proliferation has been in the border regions between Afghanistan, Pakistan and India.

The number of violent "non-state" groups has grown about 10% in the past year, according to the 2008 military balance report by the International Institute for Strategic Studies.

Iraq and India, with more than 30 active guerrilla groups each, are the most volatile countries, the report said, with the Afghan-Pakistan border and the disputed Kashmir region between India and Pakistan the worst-affected areas.